To make changes after click on save / edit button, you can use the hook_entity_presave.hook_entity_presave(Drupal\Core\Entity\EntityInterface $entity);
Example:function CUSTOM_MODULE_node_presave(Drupal\node\NodeInterface $node) {
$node->setTitle('Edited Title');
$node->set('body', 'this is the bew body');
//CAUTION : Do not save here, because it's automatic.
}
Note : Do not re save inside of hook_entity_presave().

Many thanks for posting this! Just read the Drupal 8 docs for this hook (https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21entity.api.php/function/hook_entity_presave/8.2.x) and this would probably be most people's next logical question.